WO 2004/001867 describes an example of a piezoelectric motor that provides a linear drive for an actuator. The motor uses a piezoelectric drive element that operates at high speed and with high precision. The piezoelectric motor described in WO 2004/001867 A1 comprises a stator that consists of two series-connected bending sections and a power transmission element which is mounted on the stator and transmits the bending action of the drive element to a sliding element. The drive element is aligned parallel to the sliding element and is made of an electrostrictive material, such as a piezoelectric material. These kinds of materials change their shape when exposed to an electric voltage or a magnetic field. The bending sections of the drive element are disposed symmetrical to the power transmission elements, the two bending sections performing a bending action that is similar to a traveling wave when an electric voltage is applied. The wave-like movement is transmitted via the power transmission elements to the sliding element and moves the sliding element incrementally. The movement of the drive elements is transmitted via the power transmission elements to the sliding element in that the power transmission elements and the sliding element are in frictional contact. The electromechanical motor may be used as a regulating device that achieves fast and precise lateral displacement of the sliding element and thus of an actuator connected to it. Piezoceramics have very short response times and thus very short operating times. One possible application for a friction-coupled miniature motor of this type is in a locking system, where the sliding element is used as an actuator to close a lock cylinder.
These kinds of friction-coupled linear drives and other directly coupled linear drives—in other words drives that do not involve a rotational movement being translated into a translation movement but rather in which the translation movement is directly generated—are generally used for moving an actuator in a direction of translation. Directly coupled linear drives have the disadvantage that it is possible to manipulate the movement of the actuator by applying an external force. In particular, when exposed to a shock or impact load or any other mechanical stress, such as vibration that is applied to the linear drive from an external source, the actuator may slip along the drive unit because the directly coupled linear drive has only a limited self-restraining effect. This self-restraining effect is found in the region of static friction between the sliding element and drive unit. For linear drives in which a rotational movement is translated by a thread into a translation movement, an external force varies in its effect. If the thread pitch, for example, is low, an external force, such as a shock load, immediately results in damage to the thread. If the pitch of the thread is high, a shock load results in the actuator slipping through, similar to directly coupled linear drives.
If used, for example, for actuating a lock cylinder, the fact that the actuator could be manipulated by a simple external shock load or an externally applied vibration would of course be highly disadvantageous. Safety standards require that electronic locks as well withstand shock loads of some 1500 times the acceleration of gravity (1500 g) or more. One g corresponds to an acceleration of 9.81 m/sec2.
It is an object of the invention to provide a linear drive that cannot be manipulated by the application of external forces, such as shock loads and vibrations, and at the same time can maintain its original performance.